1. Field of the Invention
This invention relates to a primer composition adaptable for both steel sheet and plastic sheet as vehicle bodies.
2. Related Art Statement
In the painting of steel sheets and plastic sheets for vehicle bodies, there has hitherto been adopted a method wherein the painting is performed on every sheet and then the painted sheets are assembled at a final step.
The steel sheet is degreased and phosphated to form a phosphate layer thereon. After an under coat is formed by electrodeposition, a chipping primer or a stoneguard coat is partially applied and then an inter coat and a top coat are formed thereon.
In the formation of the inter coat, a chipping-resistant and corrosion-resistant inter coating consisting of a film forming resin composed mainly of an acidic resin having an acid value of 10.about.50, talc powder and rust-preventive pigment is applied in order to prevent the rusting and the promotion of corrosion even if the metal substrate is partially exposed by chipping, which is disclosed in Japanese Patent laid open No. 55-56,165.
On the other hand, the plastic sheet is degreased, dried and coated with an exclusive primer for the plastic sheet. Then, an exclusive inter coat and an exclusive top coat are successively formed thereon. In this case, the formation of the exclusive inter coat may be omitted.
As the exclusive primer, Japanese Patent Application Publication No. 51-36,285 discloses a primer composed of a reaction product between ungelled hydroxy-containing urethane reactant and polycarboxylic acid or its anhydride and an aminoplast resin for polyurethane resin used as a sheet material in automobile bumpers or molded articles.
Further, Japanese Patent laid open No. 59-30,830 discloses a primer consisting mainly of chlorinated polypropylene for polypropylene resin used as a sheet material in the automobile bumper.
Assuming that polyamide resin, polybutylene terephthalate resin, polycarbonate resin or the like is used as a top coat forming a hard painted coating on the steel sheet for the vehicle body, a primer composition for plastic parts comprising a resin obtained by grafting propylene-ethylene copolymer with maleic acid or maleic anhydride and a conductive substance is used as disclosed in Japanese Patent laid open No. 61-76,557.
Thereafter, the painted plastic sheet is assembled onto the painted steel sheet, which is transferred to subsequent steps.
As the other painting method, there is a method wherein the plastic sheet is degreased, dried, coated with the above exclusive primer, baked and then assembled onto the steel sheet previously degreased, phosphated and coated with an under coat, and thereafter the common inter coat and top coat are formed and baked on the assembly of the plastic sheet and steel sheet.
According to these conventional painting methods, however, the structure of the resulting paint coats is different between the plastic sheet and the steel sheet, and the paint used and the painting step are also different, so that the following problems exit.
(1) Since the painting equipment must be separately arranged for the painting of steel sheet and for the painting of plastic sheet, not only the operation efficiency is poor but also an huge equipment investment is required;
(2) Differences in appearance such as color tone, gloss and the like are caused between the plastic sheet and the steel sheet which degrade the commercial value of the automobile as a whole;
(3) Differences in the durability of painted coatings are caused between the plastic sheet and the steel sheet to injure the beautiful appearance of the automobile from a long-term viewpoint and; (4) When the plastic sheet is coated with the exclusive primer, assembled into the steel sheet and then the resulting assembly is subjected to painting, there are caused no problems as described in the above items (1)-(3). In the selection of paints for inter coat and top coat, however, there are problems such that when a soft coating is used in conjunction with the plastic sheet, the weather resistance, polishing property, solvent resistance, stain resistance and the like are degraded, while when a hard coating is used in conjunction with the steel sheet, the impact resistance, chipping resistance and the like of the plastic sheet are degraded. Further, it has been attempted to examine various top coats suitable for both plastic sheet and steel sheet. However, there is still a problem that the performances such as polishing property, stain resistance and so on are not consistent with the performances for plastic sheet such as impact resistance, chipping resistance and so on.